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Home , Archenteron, Blastocoel, Blastoderm, Blastulation, Cleavage (embryo), Coelom

Animal Tissues & Development

Animal Development & Differentiation Tissues and Animal Cells

• Eukaryotic • No wall No plastids No central vacuole • Multicellular: – extensive specialization & differentiation – unique cell-cell junctions

Fig. 6.8

Animals & Cell junctions

• Motile Cadherins • Highly differentiated • “calcium-dependent adhesion” transmembrane tissues – -specific • Intercellular junctions • Δ [Ca++] Þ Δ adhesion strength – Allow developmental – tissue-specific cadherins cell migration • Δ type Þ Δ binding • Extracellular – Allow developmental fibers tissue separation – collagen • Diploid life cycle • Blastula/gastrula

Extracellular Matrix (ECM) post-fertilization events

1 Binding of to egg

• Collagen fibers • 2 Acrosomal reaction: plasma membrane (), a 3 depolarization (fast block to polyspermy) • Elastin fibers 4 • 6 8 – Attachment / movement along ECM 10

Seconds Increased intracellular calcium level

20 Cortical reaction begins (slow block to polyspermy)

Sperm 30 head 40 50 1 Formation of fertilization envelope complete EGG 2 Increased intracellular pH 3 4 5 Increased protein synthesis

10

Minutes 20 Fusion of egg and sperm nuclei complete

30 40 Onset of DNA synthesis 60 90 First Figure 47.5

Heyer 1 Animal Tissues & Development

Cleavage: DNA replication / / with no growth phases products of cytokinesis smaller & smaller Spiral vs. Radial • : “ first” – most S S – spiral cleavage G1 G2 M – determinate M • : “mouth second” during Cell cycle after – cleavage stage cleavage stage & – radial cleavage – indeterminate

• Little/no synthesis of new RNA or proteins • All cells dependent upon molecular machines from original ovum

Determinate cleavage in a (round worm) Determinate cleavage in a protostome (round worm)

0 Cytoplasmic determinates – RNA-protein complexes First cell division •Define cell fate and body axis.

Nervous Musculature, Outer , Germ line • E.g., “P-granules” in round worm embryo system, gonads (future outer skin, gametes) • Dispersed in muscula- ture Musculature • After fertilization, aggregates at future posterior end 20 μm • Upon each cleavage, partition to posterior-most cell

Time after fertilization (hours) 10 Hatching

Intestine

Intestine 1 Newly fertilized egg 3 Two-cell embryo

Mouth Eggs Vulva

ANTERIOR POSTERIOR 1.2 mm 2 Zygote prior to first division 4 Four-cell embryo Figure 47.19 Figure 47.21

Indeterminate cleavage in a () Experiment — Sea Urchin Control egg Experimental egg (dorsal view) (side view) • Cleavage partitions the cytoplasm of one large cell into 1a 1b Gray many smaller cells called blastomeres Gray crescent crescent • Continued cleavage è hollow structure called a blastula – The hollow cavity is the

Thread

• In general, tissue-specific fates of cells are fixed by the late gastrula stage 2

Results (a) Fertilized egg. Shown (b) Four-cell stage. (c) . After further (d) Blastula. A single layer of here is the zygote shortly Remnants of the cleavage divisions, the cells surrounds a large before the first cleavage mitotic spindle can be embryo is a multicellular blastocoel cavity. division, surrounded by seen between the two ball that is still surrounded Although not visible here, the fertilization envelope. cells that have just by the fertilization the fertilization envelope The nucleus is visible in completed the second envelope. The blastocoel is still present; the embryo the center. cleavage division. cavity has begun to form. will soon hatch from it and Normal Belly piece Normal begin swimming. Figure 47.23 Figure 47.6

Heyer 2 Animal Tissues & Development

Animal Morphogenesis Morphogenesis • Creation of form - directed by • In , by differential growth – Cell proliferation • In , by both growth & cell migration – Cell migration – Cell differentiation Cell Gut movement – Cell death (apoptosis)

Zygote Eight cells Blastula Gastrula Adult animal (fertilized egg) (cross section) (cross section) (sea star)

Cell division Morphogenesis Observable cell differentiation Seed leaves Shoot apical meristem

Root apical Zygote meristem Figure 21.4 (fertilized egg) Two cells Embryo inside seed

Blastulation & Primary embryonic germ layers • Early in animals • Diploblastic: two germ layers 3 In most animals, cleavage results in the – : develops into epidermal & neural tissues 1 The zygote of an animal formation of a multicellular stage called a blastula. The blastula of many animals is a undergoes a succession of mitotic – : develops into feeding tissues cell divisions called cleavage. hollow ball of cells. – Blastocoel: becomes filled with acellular mesoglia Blastocoel Cleavage Cleavage

Blastocoel 6 The endoderm of the Examples: develops into Eight-cell stage Blastula Cross section Zygote Endoderm the the animal’s of blastula Porifera & digestive tract. Blastocoel Endoderm

5 The blind Ectoderm pouch formed by Ectoderm gastrulation, called the archenteron, opens to the outside Gastrula Gastrulation via the blastopore. Blastopore 4 Most animals also undergo gastrulation, a rearrangement of the embryo in which one end of the embryo folds inward, expands, and eventually fills the Blastopore blastocoel, producing layers of embryonic tissues: the Figure 32.2 ectoderm (outer layer) and the endoderm (inner layer).

Primary embryonic germ layers • Triploblastic: three germ layers Triploblastic gastrulation forms – Ectoderm: develops into epidermal & neural tissues three germ layers – Endoderm: develops into gut & accessory organs ECTODERM ENDODERM – Mesoderm — displaces blastocoel: develops into • of skin and its • • Epithelial lining of muscle, connective tissues, & vasculature derivatives (including sweat • Endoskeletal system digestive tract glands, follicles) • Muscular system • Epithelial lining of • Epithelial lining of mouth • Muscular layer of and stomach, intestine, etc. • Lining of , urinary Examples: • Sense receptors in • bladder, and reproductive everything else epidermis • Circulatory and lymphatic system • Cornea and lens of systems • • Nervous system • Reproductive system • Archenteron • Adrenal medulla (except germ cells) • • enamel • of skin • and parathyroid • or pineal and • Lining of glands pituitary glands • Adrenal cortex

Figure 47.16

Mesoderm Blastopore Figure 32.10b

Heyer 3 Animal Tissues & Development

c.f., Figure 40.5 Epithelial Tissue Triploblastic Animal Tissues • Typical mammalian body is composed of ~50,000,000,000,000 cells • Continuous sheet • Typical body is composed of >100 or layers of cells specialized types of cells (tissue types) with direct cell- – Grouped into four major tissue types: cell junctions

• Epithelial • All three germ • Connective layers start as epithelia, so • Muscle epithelial tissues • Nervous may derive from any .

c.f., Figure 40.5 Connective Tissue • Cells are • Cells are suspended in an suspended in an extracellular extracellular

matrix. Epithelial tissue matrix. – often largely (Mucosa) – often largely composed of composed of collagen collagen

fibers. Connective fibers. tissue cells • Derived from • Derived from mesoderm. mesoderm.

c.f., Figure 40.5 c.f., Figure 40.5 Muscle Tissue Nervous Tissue

• Specialized to conduct electrochemical • Specialized for impulses. contraction. • Derived from Note: “Nerve” = • Derived from ectoderm. bundle of axons from mesoderm. multiple neurons Glia 15 • Diploblastic Neuron: µm Dendrites animals have Cell body Axons of myo-epithelia Axon for contraction. neurons m 40 µ (Fluorescent LM) (Confocal LM)

Heyer 4 Animal Tissues & Development

Tissues è Organs è Systems External environment

Food CO2 Mouth O2 Animal body m

Respiratory µ system tissue (SEM) 250 Interstitial fluid

Nutrients Cells Circulatory system

Digestive Excretory

m system system m µ µ 50 Lining of small 100 Anus vessels in kidney (SEM) intestine (SEM) Unabsorbed Metabolic waste matter (feces) products (nitrogenous waste)

Body Symmetry • Developmental pattern formation results in Bauplan: symmetry of growth and regional specialization Radial symmetry. The parts Ger. “Life Plan” (pl: baupläne) of a radial animal, such as a sea anemone ( Cnidaria), radiate from the center. Any imaginary slice The arrangement, pattern, and through the central axis divides the animal into development of tissues, organs, and mirror images. systems unique to a particular type of

Bilateral symmetry. A organism. bilateral animal, such as a lobster (phylum Arthropoda), has a left side and a right side. Only one imaginary cut divides the animal into mirror-image halves. Figure 32.7

(c) Variations in Acoelomate. Body covering (from ectoderm) Tissue- filled region (from Eucoelomate Gastrulation – Formation of coelom (body mesoderm) cavity) allows movement of • Coelom development in open vs. closed circulation

organs within the body, Digestive tract esp. gut expansion & motility (from endoderm) (b) Pseudocoelomate. worm Body covering • Acoelomate: no body cavity (from ectoderm)

Muscle layer • Pseudocoelomate: cavity Pseudocoelom (from between endoderm & mesoderm)

mesoderm Digestive tract (from ectoderm) • Eucoelomate: cavity within (a) Body covering mesoderm Coelomate. worm Coelom (from ectoderm)

Tissue layer lining coelom and suspending internal organs (from mesoderm) Figure 32.9 Digestive tract (from endoderm)

Heyer 5 Animal Tissues & Development

More variations in Gastrulation: Protostome development Deuterostome development (examples: molluscs, (examples: echinoderms, , ) vertebrates) Digestive tract (a) Cleavage Eight-cell stage Eight-cell stage • Gastrovascular cavity (blind gut) – Blastopore remains only orifice to gut • Protostome (“mouth first”) development Spiral and determinate Radial and indeterminate – The blastopore becomes the mouth (b) Coelom Coelom – Secondary to form anus formation • Deuterostome (“mouth second”) development Archenteron – The blastopore becomes the anus Coelom – Secondary invagination to form mouth Mesoderm Blastopore Blastopore Mesoderm Solid masses of mesoderm Folds of archenteron split and form coelom. form coelom. Anus Mouth (c) Fate of the Anus Mouth blastopore Digestive tube Gastrovascular Digestive tube Cavity Key Ectoderm Mouth Mouth Anus Mesoderm Mouth Anus Mouth develops Mouth develops Anus develops Endoderm Mouth develops from blastopore. Anus develops from blastopore. from blastopore. from blastopore. from blastopore. Figure 32.10

Protostome development Deuterostome development (examples: molluscs, (examples: echinoderms, annelids, arthropods) vertebrates) (a) Cleavage Eight-cell stage Eight-cell stage

Spiral and determinate Radial and indeterminate

(b) Coelom Coelom formation Archenteron

Coelom Mesoderm Blastopore Blastopore Mesoderm Solid masses of mesoderm Folds of archenteron split and form coelom. form coelom.

(c) Fate of the Anus Mouth blastopore Another “textbook myth”! Blastopore fate no Digestive tube Key longer considered significant to . Ectoderm Mesoderm Mouth Anus Endoderm Mouth develops from blastopore. Anus develops from blastopore.

Figure 32.10

Gastrulation — Sea Urchin Gastrulation — Sea Urchin

Yet another “textbook myth”! • Coelom formation in most deuterostomes does not derive from “folds of archenteron”.

Figure 47.8

Heyer 6 Animal Tissues & Development

Protostome Larval Development Caenorhabditis elegans ecdysozoan nematode worm Protostomal development occurs in two distinct animal groups • : have ciliated larval stages – Usually with a distinct larval stage called a trochophore Apical tuft of cilia • : have no ciliated tissues Eutelic – All stages have an external cuticle development — post-hatch growth Mouth – Growth requires ecdysis (molting) by hypertrophy only

Anus trochophore Figure 32.12

959 cells — ♀ 1031 cells — ♂ ecdysis

More Variations in Deuterostome Gastrulation Vertebrate Development

From M.K. Richardson (1997) &

Figure 47.2 EMBRYONIC DEVELOPMENT Radial Cleavage & Blastulation — Frog Sperm Zygote

Adult Egg • Large frog content necessitates asymmetrical

FERTILIZATION blastulation CLEAVAGE

ORGANOGENESISGASTRULATION Metamorphosis Blastula

Larval Gastrula stages

Tail-bud embryo

Heyer 7 Animal Tissues & Development

Zygote Animal 47-7: Frog embryo — Animal 47-22: Frog body hemisphere Point of Animal pole Cleavage planes & hemisphere polarity — established sperm entry asymmetrical Cleavage furrow during & blastulation fertilization Vegetal 2-cell 0.25 mm hemisphere stage Vegetal Gray forming Vegetal pole hemisphere crescent

Point of sperm 4-cell entry Future Animal stage dorsal Anterior pole forming side of tadpole Gray Right 0.25 mm crescent Blastocoel Ventral Dorsal First 8-cell cleavage stage

Left

Posterior Blastula Body axes Establishing the axes (cross section)

SURFACE VIEW CROSS SECTION 47-18: frog fate map Animal pole 47-10: frog 1 Blastocoel gastrulation Epidermis of blasto- Dorsal lip pore Central of blastopore Epidermis Blastopore nervous Early Vegetal pole gastrula system

2 Blastocoel shrinking Archenteron Notochord

Mesoderm

Blastopore Blastopore Endoderm 3 Ectoderm Blastocoel Mesoderm stage remnant Endoderm Blastula (transverse section) Archenteron Fate map of a frog embryo Future ectoderm Blastopore Future mesoderm Late Future endoderm gastrula Blastopore Yolk plug Coelom formation by mesoderm separation

Holoblastic cleavage Meroblastic cleavage • Complete division of the egg into blastomeres • Only non-yolk cytoplasm of the egg cleaved – Occurs in species whose eggs have little or moderate amounts of yolk – Occurs in species whose large eggs have abundant yolk • Most of yolk partitioned into vegetal pole blastomeres – Establishes anterior/posterior axis Development

50 μm Sea urchin: •small egg; little yolk •Symmetrical holoblastic cleavage

Frog: •larger egg; moderate yolk •Asymmetrical holoblastic cleavage 250 μm Source: http://www.ucalgary.ca/UofC/eduweb/virtualembryo/why_fish.html

Heyer 8 Animal Tissues & Development

Extreme asymmetric blastulation in many vertebrates

Fertilized egg Figure 47.10 Fish • Large, yolk-rich eggs Disk of Development cytoplasm • Extreme meroblastic 1 Zygote. Most of the cell’s volume is cleavage forms the yolk, with a small disk of cytoplasm located . at the animal pole. 2 • Separation of the Four-cell stage. from the forms the 3 Blastoderm. The many cleavage blastocoel. divisions produce the blastoderm, a mass of cells that rests on top of the yolk mass.

Cutaway view of the blastoderm. The cells of the Blastocoel BLASTODERM blastoderm are arranged in two layers, the epiblast and YOLK MASS hypoblast, that enclose a fluid- Epiblast Hypoblast filled cavity, the blastocoel. Source: http://www.ucalgary.ca/UofC/eduweb/virtualembryo/why_fish.html

Gastrulation — Chick Gastrulation — Chick • Instead of blastopore, groove () forms in blastoderm. • from germ layers. • All three germ layers form from infolding epiblast.

Eye Forebrain Epiblast Neural tube Notochord Heart Coelom Future Primitive Archenteron ectoderm Lateral fold Endoderm Blood streak Mesoderm vessels Ectoderm

Yolk stalk Migrating Form extraembryonic Neural tube cells Endoderm membranes YOLK (mesoderm) Hypoblast (a) Early organogenesis. The archenteron forms when (b) Late organogenesis. 56 hours old lateral folds pinch the embryo away from the yolk. chick embryo, about 2–3 mm long (LM). YOLK

Figure 47.11 Figure 47.15

Amniotes: extra-embryonic membranes embryo & extraembryonic membranes Embryo

Amniotic Albumen cavity with amniotic fluid

Shell

Yolk (nutrients) Yolk sac

47-13: chick extra-embryonic membranes

Heyer 9